The present invention is generally in the area of transgenic animal models of atherosclerosis and methods for screening for inhibitors acting via interaction with the SR-BI scavenger receptor.
The intercellular transport of lipids through the circulatory system requires the packaging of these hydrophobic molecules into water-soluble carriers, called lipoproteins, and the regulated targeting of these lipoproteins to appropriate tissues by receptor-mediated pathways. The most well characterized lipoprotein receptor is the LDL receptor, which binds to apolipoproteins B-100 (apoB-100) and E (apoE), which are constituents of low density lipoprotein (LQL), the principal cholesteryl-ester transporter in human plasma, very low- density lipoprotein (VLDL), a triglyceride-rich carrier synthesized by the liver, intermediate-density lipoprotein (IDL), and catabolized chylomicrons (dietary triglyceride-rich carriers).
All members of the LDL receptor gene family consist of the same basic structural motifs. Ligand-binding (complement-type) cysteine-rich repeats of approximately 40 amino acids are arranged in clusters (ligand-binding domains) that contain between two and eleven repeats. Ligand-binding domains are always followed by EGF-precursor homologous domains. In these domains, two EGF-like repeats are separated from a third EGF-repeat by a spacer region containing ihe YWTD motif. In LRP and gp330, EGF-precursor homologous domains are either followed by another ligand-binding domain or by a spacer region. The EGF-precursor homology domain, which precedes the plasma membrane, is separated from the single membrane-spanning segment either by an O-linked sugar domain (in the LDL receptor and VLDL receptor) or by one (in C. elegans and gp330) or six EGF-repeats (in LRP). The cytoplasmic tails contain between one and three xe2x80x9cNPXYxe2x80x9d internalization signals required for clustering of the receptors in coated pits. In a later compartment of the secretory pathway, LRP is cleaved within the eighth EGF-precursor homology domain. The two subunits LRP-515 and LRP-85 (indicated by the brackets) remain tightly and non-covalently associated. Only partial amino acid sequence of the vitellogenin receptor and of gp330 are available.
LDL receptors and most other mammalian cell-surface receptors that mediate binding and, in some cases, the endocytosis, adhesion, or signaling exhibit two common ligand-binding characteristics: high affinity and narrow specificity. However, two additional lipoprotein receptors have been identified which are characterized by high affinity and broad specificity: the macrophage scavenger receptors class A type I and type II.
Scavenger receptors mediate the endocytosis of chemically modified lipoproteins, such as acetylated LDL (AcLDL) and oxidized LDL (OxLDL), and have been implicated in the pathogenesis of atherosclerosis (Krieger and Herz, 1994 Annu. Rev. Biochem. 63, 601-637; Brown and Goldstein, 1983 Annu. Rev. Biochem. 52, 223-261; Steinberg et al., 1989 N. Engl. J. Med. 320, 915-924). Macrophage scavenger receptors exhibit complex binding properties, including inhibition by a wide variety of polyanions, such as maleylated BSA (M-BSA) and certain polynucleotides and polysaccharides, as well as unusual ligand-cross competition (Freeman et al., 1991 Proc. Natl. Acad Sci. U.S.A. 88, 4931-4935, Krieger and Herz, 1994). Several investigators have suggested that there may be at least three different classes of such receptors expressed on mammalian macrophages, including receptors which recognize either AcLDL or OxLDL, or both of these ligands (Sparrow et al., 1989. J Biol. Chem 264, 2599-2604; Arai et al., 1989 Biochem. Biophys. Res. Commun. 159, 1375-1382; Nagelkerke et al., 1983 J Biol. Chem. 258, 12221-12227).
The first macrophage scavenger receptors to be purified and cloned were the mammalian class A type I and II receptors. These are trimeric integral membrane glycoproteins whose extracellular domains have been predicted to include a-helical coiled-coil, collagenous and globular structures (Kodama et al., 1990 Nature 343, 531-535; Rohrer et al., 1990 Nature 343, 570-572; Krieger and Herz, 1994). The collagenous domain, shared by the class A type I and type II receptors, apparently mediates the binding of polyanionic ligands (Acton et al., 1993 J Biol. Chem. 268, 3530-3537; Doi et al., 1993 J Biol. Chem. 268, 2126-2133). The class A type I and type II molecules, which are the products of alternative splicing of a single gene, are hereafter designated class A-scavenger receptors (SR-Al and SR-All). The class A receptors, which bind both AcLDL and OxLDL (Freeman et al., 1991), have been proposed to be involved in host defense and cell adhesion, as well as atherogenesis (Freeman et al., 1991; Krieger, 1992 Trends Biochem. Sci. 17, 141-146; Fraser et al., 1993 Nature 364, 343-346; Krieger and Herz, 1994).
Based on models of the predicted quaternary structures of the class A type I and type II macrophage scavenger receptors, both contain six domains, of which the first five are identical: the N-terminal cytoplasmic region, the transmembrane region, spacer, a-helical coil, and collagen-like domains. The C-terminal sixth domain of the type I receptor is composed of an eight-residue spacer followed by a 102-amino acid cysteine-rich domain (SRCR), while the sixth domain of the type II receptor is only a short oligopeptide.
Using a murine macrophage cDNA library and a COS cell expression cloning technique, Endemann, Stanton and colleagues, (Endemann, et al. 1993 J. Biol. Chem. 268, 11811-11816; Stanton, et al. J. Biol. Chem. 267, 22446-22451), reported the cloning of cDNAs encoding two additional proteins that can bind OxLDL. The binding of OxLDL to these proteins was not inhibited by AcLDL. These proteins are FcgRII-B2 (an Fc receptor) (Stanton et al., 1992) and CD36 (Endemann et al., 1993). The significance of the binding of OxLDL to FcgRII-B2 in transfected COS cells is unclear because FcgRII-B2 in macrophages apparently does not contribute significantly to OxLDL binding (Stanton et al., 1992). However, CD36 may play a quantitatively significant role in OxLDL binding by macrophages (Endemann et al., 1993). In addition to binding oxidized LDL, CD36 binds thrombospondin (Asch et al., 1987 J Clin. Invest. 79, 1054-1061), collagen (Tandon et al., 1989J Biol. Chem. 264, 7576-7583), long-chain fatty acids (Abumrad et al., 1993 J Biol. Chem. 268, 17665-17668) and Plasmodium falciparum infected erythrocytes (Oquendo et al., 1989 Cell 58, 95-101). CD36 is expressed in a variety of tissues, including adipose, and in macrophages, epithelial cells, monocytes, endothelial cells, platelets, and a wide variety of cultured lines (Abumrad et al., 1993; and see Greenwalt et al., 1992 Blood 80, 1105-1115 for review). Although the physiologic functions of CD36 are not known, it may serve as an adhesion molecule due to its collagen-binding properties. It is also been proposed to be a long-chain fatty acid transporter (Abumrad et al., 1993) and a signal transduction molecule (Ockenhouse et al., 1989 J Clin. Invest. 84,468-475; Huang et al., 1991 Proc. Natl. Acad. Sci. USA 88, 7844-7848), and may serve as a receptor on macrophages for senescent neutrophils (Savill et al., 1991 Chest 99, 7 (suppl)).
Modified lipoprotein scavenger receptor activity has also been observed in endothelial cells (Arai et al., 1989; Nagelkerke et al., 1983; Brown and Goldstein, 1983; Goldstein et al., 1979 Proc. Natl. Acad. Sci. U.S.A. 76, 333-337). At least some of the endothelial cell activity apparently is not mediated by the class A scavenger receptors (Bickel et al., 1992 J Clin. Invest. 90, 1450-1457; Arai et al., 1989; Nagelkerke et al., 1983; Via et al., 1992 The Faseb J. 6, A371), which are often expressed by macrophages (Naito et al., 1991 Am. J. Pathol. 139, 1411-1423; Krieger and Herz, 1994). In vivo and in vitro studies suggest that there may be scavenger receptor genes expressed in endothelial cells and macrophages which differ from both the class A scavenger receptors and CD36 (Haberland et al., 1986 J. Clin. Inves. 77, 681-689; Via et al., 1992; Sparrow et al., 1989; Horiuchi et al., 1985 J Biol. Chem. 259, 53-56; Arai et al., 1989; and see below). Via, Dressel and colleagues (Ottnad et al., 1992 Biochem J. 281, 745-751) and Schnitzer et al. 1992 J. Biol. Chem. 267, 24544-24553) have detected scavenger receptor-like binding by relatively small membrane associated proteins of 15-86 kD. In addition, the LDL receptor related protein (LRP) has been shown to bind lipoprotein remnant particles and a wide variety of other macromolecules. Both the MRNA encoding LRP and the LRP protein are found in many tissues and cell types (Herz, et al., 1988 EMBO J. 7:4119-4127; Moestrup, et al., 1992 Cell Tissue Res. 269:375-382), primarily the liver, the brain and the placenta. The predicted protein sequence of the LRP consists of a series of distinctive domains or structural motifs, which are also found in the LDL receptor.
As described by Kreiger, et al., in PCT/US95/07721 xe2x80x9cClass BI and CI Scavenger Receptorsxe2x80x9d Massachusetts Institute of Technology (xe2x80x9cKrieger, et al.xe2x80x9d), two distinct scavenger receptor type proteins having high affinity for modified lipoproteins and other ligands have been isolated, characterized and cloned. Hamster and murine homologs of SR-BI, an AcLDL and LDL binding scavenger receptor, which is distinct from the class A type I and type II macrophage scavenger receptors, has been isolated and characterized. In addition, DNA encoding the receptor cloned from a variant of Chinese Hamster Ovary Cells, designated Var-261, has been isolated and cloned. dSR-CI, a non-mammalian AcLDL binding scavenger receptor having high ligand affinity and broad specificity, was isolated from Drosophila melanogaster.
It was reported by Kreiger, et al. that the SR-BI receptor is expressed principally in steroidogenic tissues and liver and appears to mediate HDL-transfer and uptake of cholesterol. Competitive binding studies show that SR-BI binds LDL, modified LDL, negatively charged phospholipid, and HDL. Direct binding studies show that SR-BI expressed in mammalian cells (for example, a varient of CHO cells) binds HDL, without cellular degradation of the HDL-apoprotein, and lipid is accumulated within cells expressing the receptor. These studies indicate that SR-BI might play a major role in transfer of cholesterol from peripheral tissues, via HDL, into the liver and steroidogenic tissues, and that increased or decreased expression in the liver or other tissues may be useful in regulating uptake of cholesterol by cells expressing SR-BI, thereby decreasing levels in foam cells and deposition at sites involved in atherogenesis.
Atherosclerosis is the leading cause of death in western industrialized countries. The risk of developing atherosclerosis is directly related to plasma levels of LDL cholesterol and inversely related to HDL cholesterol levels. Over 20 years ago, the pivotal role of the LDL receptor in LDL metabolism was elucidated by Goldstein, et al., in the Metabolic and Molecular Bases of Inherited Disease, Scriver, et al. (McGraw-Hill, NY 1995), pp. 1981-2030. In contrast, the cellular mechanisms responsible for HDL metabolism are still not well defined. It is generally accepted that HDL is involved in the transport of cholesterol from extrahepatic tissues to the liver, a process known as reverse cholesterol transport, as described by Pieters, et al., Biochim. Biophys. Acta 1225, 125 (1994), and mediates the transport of cholesteryl ester to steroidogenic tissues for hormone synthesis, as described by Andersen and Dietschy, J Biol. Chem. 256, 7362 (1981). The mechanism by which HDL cholesterol is delivered to target cells differs from that of LDL. The receptor-mediated metabolism of LDL has been thoroughly described and involves cellular uptake and degradation of the entire particle. In contrast, the receptor-mediated HDL metabolism has not been understood as well. Unlike LDL, the protein components of HDL are not degraded in the process of transporting cholesterol to cells. Despite numerous attempts by many investigators, the cell-surface protein(s) that participate in the delivery of cholesterol from HDL to cells had not been identified before the discovery that SR-BI was an HDL receptor.
It is an object of the present invention to provide methods and reagents for designing drugs that can stimulate or inhibit the binding to and lipid movements mediated by SR-BI and redirect uptake and metabolism of lipids and cholesterol by cells.
Transgenic animals that do not express functional SR-BI and ApoE develop severe atherosclerosis, by age four weeks in transgenic mice. Moreover, these animals exhibit progressive heart block by age four weeks, and die by age nine weeks. Pathology shows extensive fibrosis of the heart and occlusion of coronary arteries. The occlusion appears to be due to clotting, since fat deposition is in the walls. Equivalent animals can be produced using single knockout animals with an inhibitor, for example, an inhibitor of SR-BI administered to an ApoE knockout, or verse versa. These animals are good models for the following diseases, and for screening of drugs useful in the treatment and/or prevention of these disorders: cardiac fibrosis, myocardial infarction, defects in electrical conductance, atherosclerosis, unstable plaque, and stroke. In contrast to other known models for atherosclerosis, these animals do not have to be fed extreme diets for long periods before developing atherosclerosis. No other known model for heart attacks and stroke is known.
The role of SR-BI has now been confirmed as the principle mediator of cholesteryl ester transport from peripheral tissues to the liver and other steroidogenic tissues, including the adrenal gland, testes and ovaries. The studies described herein demonstrate that animals which are deficient in both SR-BI and ApoE are not only excellent models for atheroslerosis but also myocardial infarction and stroke, since the animals develope progressive heart block and coronary artery occlusions characterized by clots resembling those in heart attack patients.
These animals can be used to screen for drugs that are effective as therapeutics or diagnostics of heart disease.
Compounds are preferably administered in a pharmaceutically acceptable vehicle. Suitable pharmaceutical vehicles are known to those skilled in the art. For parenteral administration, the compound will usually be dissolved or suspended in sterile water or saline. For enteral administration, the compound will be incorporated into an inert carrier in tablet, liquid, or capsular form. Suitable carriers may be starches or sugars and include lubricants, flavorings, binders, and other materials of the same nature. The compounds can also be administered locally by topical application of a solution, cream, gel, or polymeric material (for example, a Pluronic(trademark), BASF).
Alternatively, the compound may be administered in liposomes or microspheres (or microparticles). Methods for preparing liposomes and microspheres for administration to a patient are known to those skilled in the art. U.S. Pat. No. 4,789,734 describe methods for encapsulating biological materials in liposomes. Essentially, the material is dissolved in an aqueous solution, the appropriate phospholipids and lipids added, along with surfactants if required, and the material dialyzed or sonicated, as necessary. A review of known methods is by G. Gregoriadis, Chapter 14. xe2x80x9cLiposomesxe2x80x9d, Drug Carriers in Biology and Medicine pp. 287-341 (Academic Press, 1979). Microspheres formed of polymers or proteins are well known to those skilled in the art, and can be tailored for passage through the gastrointestinal tract directly into the bloodstream. Alternatively, the compound can be incorporated and the microspheres, or composite of microspheres, implanted for slow release over a period of time, ranging from days to months. See, for example, U.S. Pat. No. 4,906,474, 4,925,673, and 3,625,214.
The pharmaceutical compositions are administered in an effective amount effective to modify or treat the disorder. These are readily determined by measuring blood, urine and/or tissue samples using clinically available tests. The exact dosages can be determined based on the use of animal models which are accepted as predictive of the effects of drugs on steroid levels, for example, of contraceptives or cortisone.
With the knowledge of the cDNA encoding SR-BI and regulatory sequences regulating expression thereof, it is possible to generate transgenic animals, especially rodents, for testing the compounds which can alter SR-BI expression, translation or function in a desired manner. This procedure for transient overexpression in animals following infection with adenoviral vectors is described below in the examples.
There are basically two types of animals which are useful: those not expressing functional SR-BI, which are useful for testing of drugs which may work better in combination with an inhibitor of SR-BI to control levels of lipid, cholesterol, lipoprotein or components thereof, and those which overexpress SR-BI, either in those tissues which already express the protein or in those tissues where only low levels are naturally expressed.
The animals in the first group are preferably made using techniques that result in xe2x80x9cknocking outxe2x80x9d of the gene for SR-BI, although in the preferred case this will be incomplete, either only in certain tissues, or only to a reduced amount. These animals are preferably made using a construct that includes complementary nucleotide sequence to the SR-BI gene, but does not encode functional SR-BI, and is most preferably used with embryonic stem cells to create chimeras. Animals which are heterozygous for the defective gene can also be obtained by breeding a homozygote normal with an animal which is defective in production of SR-BI. These animals can then be crossed with other transgenic or knockout animals, as described in the following examples. Equivalent animals can be produced using single knockout animals with an inhibitor, for example, an inhibitor of SR-BI administered to an ApoE knockout, or verse versa.
The animals in the second group are preferably made using a construct that includes a tissue specific promoter, of which many are available and described in the literature, or an unregulated promoter or one which is modified to increase expression as compared with the native promoter. The regulatory sequences for the SR-BI gene can be obtained using standard techniques based on screening of an appropriate library with the cDNA encoding SR-BI. These animals are most preferably made using standard microinjection techniques.
These manipulations are performed by insertion of cDNA or genomic DNA into the embryo using microinjection or other techniques known to those skilled in the art such as electroporation, as described below. The DNA is selected on the basis of the purpose for which it is intended: to inactivate the gene encoding an SR-BI or to overexpress or express in a different tissue the gene encoding SR-BI. The SR-BI encoding gene can be modified by homologous recombination with a DNA for a defective SR-BI, such as one containing within the coding sequence an antibiotic marker, which can then be used for selection purposes.
Animals suitable for transgenic experiments can be obtained from standard commercial sources. These include animals such as mice and rats for testing of genetic manipulation procedures, as well as larger animals such as pigs, cows, sheep, goats, and other animals that have been genetically engineered using techniques known to those skilled in the art. These techniques are briefly summarized below based principally on manipulation of mice and rats.
The procedures for manipulation of the embryo and for microinjection of DNA are described in detail in Hogan et al. Manipulating the mouse embryo, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1986), the teachings of which are incorporated herein. These techniques are readily applicable to embryos of other animal species, and, although the success rate is lower, it is considered to be a routine practice to those skilled in this art.
Female animals are induced to superovulate using methodology adapted from the standard techniques used with mice, that is, with an injection of pregnant mare serum gonadotrophin (PMSG; Sigma) followed 48 hours later by an injection of human chorionic gonadotrophin (hCG; Sigma). Females are placed with males immediately after hCG injection. Approximately one day after hCG, the mated females are sacrificed and embryos are recovered from excised oviducts and placed in Dulbecco""s phosphate buffered saline with 0.5% bovine serum albumin (BSA; Sigma). Surrounding cumulus cells are removed with hyaluronidase (1 mg/ml). Pronuclear embryos are then washed and placed in Earle""s balanced salt solution containing 0.5% BSA (EBSS) in a 37.5xc2x0 C. incubator with a humidified atmosphere at 5% CO2, 95% air until the time of injection.
Randomly cycling adult females are mated with vasectomized males to induce a false pregnancy, at the same time as donor females. At the time of embryo transfer, the recipient females are anesthetized and the oviducts are exposed by an incision through the body wall directly over the oviduct. The ovarian bursa is opened and the embryos to be transferred are inserted into the infundibulum. After the transfer, the incision is closed by suturing.
Introduction of cDNA into ES cells:
Methods for the culturing of ES cells and the subsequent production of transgenic animals, the introduction of DNA into ES cells by a variety of methods such as electroporation, calcium phosphate/DNA precipitation, and direct injection are described in detail in Teratocarcinomas and embryonic stem cells, a practical approach, ed. E.J. Robertson, (IRL Press 1987), the teachings of which are incorporated herein. Selection of the desired clone of transgene-containing ES cells is accomplished through one of several means. In cases involving sequence specific gene integration, a nucleic acid sequence for recombination with the SR-BI gene or sequences for controlling expression thereof is co-precipitated with a gene encoding a marker such as neomycin resistance. Transfection is carried out by one of several methods described in detail in Lovell-Badge, in Teratocarcinomas and embryonic stem cells, a practical approach, ed. E.J. Robertson, (IRL Press 1987) or in Potter et al Proc. Natl. Acad. Sci. USA 81, 7161 (1984). Calcium phosphate/DNA precipitation, direct injection, and electroporation are the preferred methods. In these procedures, a number of ES cells, for example, 0.5xc3x97106, are plated into tissue culture dishes and transfected with a mixture of the linearized nucleic acid sequence and 1 mg of pSV2neo DNA (Southern and Berg, J Mol. Appl. Gen. 1:327-341 (1982)) precipitated in the presence of 50 mg lipofectin in a final volume of 100 xcexcl. The cells are fed with selection medium containing 10% fetal bovine serum in DMEM supplemented with an antibiotic such as G418 (between 200 and 500 pg/ml). Colonies of cells resistant to G418 are isolated using cloning rings and expanded. DNA is extracted from drug resistant clones and Southern blotting experiments using the nucleic acid sequence as a probe are used to identify those clones carrying the desired nucleic acid sequences. In some experiments, PCR methods are used to identify the clones of interest.
DNA molecules introduced into ES cells can also be integrated into the chromosome through the process of homologous recombination, described by Capecchi, (1989). Direct injection results in a high efficiency of integration. Desired clones are identified through PCR of DNA prepared from pools of injected ES cells. Positive cells within the pools are identified by PCR subsequent to cell cloning (Zimmer and Gruss, Nature 338, 150-153 (1989)). DNA introduction by electroporation is less efficient and requires a selection step. Methods for positive selection of the recombination event (i.e., neo resistance) and dual positive-negative selection (i.e., neo resistance and ganciclovir resistance) and the subsequent identification of the desired clones by PCR have been described by Joyner et al., Nature 338, 153-156 (1989) and Capecchi, (1989), the teachings of which are incorporated herein.
Naturally cycling or superovulated females mated with males are used to harvest embryos for the injection of ES cells. Embryos of the appropriate age are recovered after successful mating. Embryos are flushed from the uterine horns of mated females and placed in Dulbecco""s modified essential medium plus 10% calf serum for injection with ES cells. Approximately 10-20 ES cells are injected into blastocysts using a glass microneedle with an internal diameter of approximately 20 xcexcm.
Randomly cycling adult females are paired with vasectomized males. Recipient females are mated such that they will be at 2.5 to 3.5 days post-mating (for mice, or later for larger animals) when required for implantation with blastocysts containing ES cells. At the time of embryo transfer, the recipient females are anesthetized. The ovaries are exposed by making an incision in the body wall directly over the oviduct and the ovary and uterus are externalized. A hole is made in the uterine horn with a needle through which the blastocysts are transferred. After the transfer, the ovary and uterus are pushed back into the body and the incision is closed by suturing. This procedure is repeated on the opposite side if additional transfers are to be made.
Samples (1-2 cm of mouse tails) are removed from young animals. For larger animals, blood or other tissue can be used. To test for chimeras in the homologous recombination experiments, i.e., to look for contribution of the targeted ES cells to the animals, coat color has been used in mice, although blood could be examined in larger animals. DNA is prepared and analyzed by both Southern blot and PCR to detect transgenic founder (F0) animals and their progeny (F1 and F2).
Once the transgenic animals are identified, lines are established by conventional breeding and used as the donors for tissue removal and implantation using standard techniques for implantation into humans.